1. Field of the Invention
The present invention relates to a cable for high voltage direct current transmission, and to the insulating composition used therein.
More particularly, the present invention relates to a cable for high voltage direct current transmission, suitable for either terrestrial or submarine installations, comprising a conductor and an extruded insulating layer consisting of a polymer composition comprising a polyethylene and at least one unsaturated fatty acid.
The present invention also relates to an insulating composition comprising a polyethylene and at least one unsaturated fatty acid.
In the present description and in the claims, the term “high voltage” denotes a voltage in excess of 35 kV.
2. Description of the Related Art
For high voltage direct current transmission, either along terrestrial lines or, in particular, along submarine lines, use is generally made of cables commonly known in the art as mass-impregnated cables, in which the conductor, covered with a first semiconducting layer, is electrically insulated by taping with an insulating material, generally paper or paper-polypropylene-paper multi-layer laminates, which is then thoroughly impregnated with a mixture having high electrical resistivity and high viscosity, generally a hydrocarbon oil to which a viscosity-increasing agent has been added. The cable additionally comprises a further semiconducting layer and a metal screen, generally made from lead, which in turn is surrounded by at least one metal armouring structure and by one or more protective sheaths of plastic material.
Mass impregnated cables, although characterized by high reliability in operation even with very high voltages (in excess of 150 kV) have certain drawbacks, principally related to the migration of insulating fluid within the cable. In particular, when in use the cable is subjected, owing to variations in the intensity of the current carried, to thermal cycles which cause migrations of the fluid in the radial direction. This is because, when the current carried increases and the cable heats up, the viscosity of the insulating fluid decreases and the fluid is subjected to a thermal expansion greater than that of all the other elements of the cable. This results in a migration of the fluid from the insulating layer towards the exterior, and consequently an increase of the pressure exerted on the metal screen which is deformed in the radial direction. When the current carried decreases and the cable cools, the impregnating fluid contracts, while the metal screen, being constituted by a plastic material (usually lead), remains permanently deformed. Thus a decrease of the pressure within the cable is caused, and this leads to the formation of micro-cavities in the insulating layer, with a consequent risk of electrical discharges and therefore of perforation of the insulation. The risk of perforation increases with an increase in the thickness of the insulating layer, and therefore with an increase in the maximum voltage for which the cable has been designed.
Another solution for high voltage direct current transmission consists in the use of fluid oil cables, in which the insulation is provided by a pressurized oil with low viscosity and high electrical resistivity which is (under hydrostatic head). Although this solution is highly effective in preventing the formation of micro-cavities in the cable insulation, it has various drawbacks, mainly related to the complexity of construction and, in particular, it imposes a limitation on the maximum permissible length of the cable. This limitation on the maximum length is a major drawback, particularly in the case of use in submarine installations, where the lengths required are usually very great.
For many years, research has been directed towards the possibility of using cross-linked polyolefins, and, particularly, cross-linked polyethylene (XLPE), to produce insulating materials for cables for direct current transmission. Insulating materials of this type are already widely used in cables for alternating current transmission. The use of said insulating materials also in the case of cables for direct current transmission would allow the said cables to be used at higher temperatures, for example at 90° C. instead of 50° C., than those at which the previously described mass impregnated cables can be used (higher operating temperatures allow the quantity of current transported to be increased), and would also remove the limitations on the maximum permissible length of the cable, by contrast with the case of the fluid oil cables described above.
However, it has not yet been possible to make adequate and complete use of said insulating materials, particularly for direct current transmission. The common view is that one of the main reasons for this limitation is the development and accumulation of what are called space charges in the dielectric insulating material when the said material is subjected to direct current. It is considered that the space charges alter the distribution of the electrical field and persist for long periods because of the high resistivity of the polymers used. The accumulation of the space charges leads to a local increase of the electrical field which consequently comes to be greater than that which would be expected on the basis of the geometrical dimensions and dielectric characteristics of the insulating material.
The accumulation of the space charges is a slow process: however, the problem is accentuated if the direct current transported by the cable is reversed (in other words, if there is a reversal of polarity). As a result of this reversal, a capacitive field is superimposed on the overall electrical field and the value of the maximum gradient can be localized within the insulating material.
It is known that a prolonged degassing treatment which can be carried out, for example, by subjecting the insulating material based on a cross-linked polymer to high temperatures and/or to a high vacuum for a long period, can be used to obtain an insulating material capable of limiting the accumulation of the space charges when the cable is subjected to polarity reversal. In general, it is believed that the said degassing treatment reduces the formation of the space charges as a result of the removal of the decomposition products of the crosslinking agent (for example, dicumyl peroxide, which decomposes to form acetophenone and cumyl alcohol) from the insulating material. However, a prolonged degassing treatment evidently leads to an increase in production times and costs.
There is a known way of attempting to reduce the accumulation of space charges by modifying the crosslinked polyethylene (XLPE) by introducing small quantities of polar groups.
For example, Japanese patent application JP-A210610 describes a cross-linked polyethylene, modified by grafting maleic anhydride in a quantity of between 0.02% and 0.5% by weight, which would be usable as an insulating material for cables for direct current transmission, since it would be capable of trapping the space charges and thus reducing their accumulation.
Japanese patent application JP 10/283,851 describes a cable for direct current transmission with improved dielectric strength in the presence of polarity reversals or following the applications of electrical pulses, in which the insulating layer consists of a polymeric composition comprising a cross-linked polyolefin containing (i) a dicarboxylic acid anhydride and (ii) at least one monomer containing a polar group (chosen from at least one carbonyl, nitrile or nitro group). However, this requires the presence of a particular peroxide, more precisely 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane, and of a particular antioxidant, more precisely an ester of a thiocarboxylic acid.
Patent application EP 0 463 402 describes an ethylene (co)polymer containing polar groups chosen from ketone, nitrile and nitro groups in quantities ranging from 20 ppm to 8000 ppm, the said polar groups having a dipole moment of more than 0.8 debye. The said (co)polymer would be usable as an insulating material for high-voltage cables having an improved dielectric strength.
Patent application WO 99/405589 relates to a cable for direct current transmission in which the insulating layer consists of cross-linked polyethylene comprising polar groups obtained by pretreatment of the polyethylene with molecular oxygen before extrusion.
Patent application WO 99/44207 relates to a cable for direct current transmission in which the insulating layer consists of a polymeric composition based on cross-linked polyethylene modified by polar groups. The said polar groups, having the general formula:CH2═CR—CO—X—(CH2)n—N(CH3) 2 or CH2═CR—CO—O—(CH2—CH2O)n—H,in which n is 2 or 3, m is a number ranging from 1 to 20, R is H or CH3, and X is O or NH, are introduced into the cross-linked polyethylene by copolymerization or grafting. Examples of the said polar groups are dialkyl-aminopropyl-(met)acrylamide or (oligo)-ethyleneglycol-methacrylate.
Japanese patent application JP 06/215645 describes a cable for high voltage direct current transmission with reduced accumulation of space charges. The insulating layer is made by hot crosslinking of a mixture of polyethylene, an organic peroxide having a half-life time of more than 5 hours at 130° C., and an acid chosen from itaconic acid and crotonic acid in a quantity of less than 5 parts by weight per 100 parts by weight of polyethylene.
Patent application WO 00/08655 relates to a cable for direct current transmission in which the insulating layer consists of a polymeric composition based on polyethylene to which an esterified (poly)glycerol having at least two free OH groups is added.